The present invention relates generally to an airfoil blade for turbines, compressors, fans, and the like, and more particularly to such an airfoil blade having improved internal cooling.
Turbines have airfoil-shaped blades which are employed to extract useful work from a hot moving fluid. Jet engines include turbines which have some turbine blades attached to a rotating turbine rotor disc and some turbine blades (sometimes called vanes) attached to a non-rotating turbine stator casing. Modern aircraft jet engines have employed internal cooling of turbine blades to keep blade temperatures within design limits. Typically, such blades are cooled by air passing through internal passages, with the air entering near the blade root (the attached portion of the blade) and exiting near the blade tip (the free end portion of the blade). Known turbine blade cooling passages having a serpentine or helical shape improve cooling effectiveness by increasing the length of the coolant flow path beyond the path length for straight-line flow. A serpentine-shaped passage produces serpentine flow and has a serpentine-shaped centerline running within and down the middle of the passage, while a helical-shaped passage produces helical flow and has a helical-shaped centerline running within and down the middle of the passage. On the one hand, there is a thermal stress problem with a blade having a serpentine passage because the coolant continuously picks up heat as it travels up and down the blade which creates a non-uniform temperature across the blade from its leading edge to its trailing edge. On the other hand, a blade having a helical passage over-cools the unused central area about which the helical passage spirals, again leading to a non-uniform temperature across the blade.